Nowadays, a liquid crystal display has been used commonly in personal computers, PDAs (persona digital assistants), mobile phones, and the like. As such devices become widespread, the chances of displaying information on a liquid crystal display screen in public are increasing.
The following is a brief description of a configuration of a conventional liquid crystal display. The liquid crystal display has a liquid crystal layer sandwiched between a pair of translucent substrates. On one of the translucent substrates i.e., an active matrix substrate), data signal lines to which display data is applied and gate signal lines for driving active elements described below are arranged in matrix. At each intersection of the data signal lines and the gate signal lines, the active element such as a thin film transistor (TFT) to which a picture element electrode is connected is connected. The picture element electrodes are arranged on the translucent substrate in matrix. On the other translucent substrate (i.e., a counter substrate), a color filter layer including color filters of red, green, and blue, and a black matrix for preventing light leakage between these color filters is formed. On the color filter layer, a transparent conductive film functioning as a counter electrode for driving liquid crystal is laminated.
Further, on each of the active matrix substrate and the counter substrate, a Liquid crystal alignment film made of polyimide or the like is formed under a predetermined condition, followed by an alignment treatment such as rubbing in accordance with a desired viewing angle. Then, a spacer such as plastic beads for forming a cell gap is arranged on one of the substrates, and an adhesive such as a thermosetting sealant is applied to the periphery of a display area on the other substrate. Thereafter, the active matrix substrate and the counter substrate are adhered to each other with the adhesive, and a liquid crystal material is injected between the substrates by a vacuum injection method or the like, followed by sealing of an injection port. In this manner a liquid crystal display element is obtained.
Meanwhile, a liquid crystal display is required to have display quality such as high brightness, high contrast, high definition, and a wide viewing angle, which is being improved rapidly. In particular, conventional modes such as a TN (twisted nematic) mode and a STS (super twisted nematic) mode are disadvantageous in that they lead to a narrow viewing angle. In order to overcome this disadvantage, liquid crystal modes capable of providing a wide viewing angle, such as MVA (multi-domain vertical alignment), CPA (continuous pinwheel alignment), IPS (in-plane switching), FFS (fringe field switching), OCB (optically compen stated birefringence), are being developed. However, the development of such liquid crystal modes capable of providing a wide viewing angle has led to another problem.
That is, when a user observes a screen of a mobile personal computer, a PDA, a mobile phone, or the like having a wide viewing angle in public, the wide viewing angle of the screen allows others to view the screen from a viewing angle in a direction other than a front direction for the user, such as a horizontal direction, which may result in leakage of private information. Nowadays, combined with enforcement of the Personal Information Protection Law, there has been an increasing demand from users for privacy protection by making a viewing angle narrower contrary to a conventional tendency. To this end, it has been popular to attach a viewing angle adjusting film for making a viewing angle narrower to a screen, which, however, always makes the viewing angle narrower. Thus, it also has been demanded to switch dynamically between a wide viewing angle inherent in a display and a narrow viewing angle in favor of privacy protection according to need. In view of this, a display in which a viewing angle control device capable of switching dynamically between a wide viewing angle and a narrow viewing angle by using birefringence of liquid crystal is provided on a display device for displaying an image has been proposed.
Various systems of viewing angle control devices using liquid crystal have been proposed, such as a light dispersion system using polymer-dispersed liquid crystal (Japanese Patent No. 3481741 and JP 7(1995)-333640 A), a light absorption system using guest host liquid crystal (JP 10(1998)-197844 A), a TN mode (JP 10(1998)-268251 A), an IPS mode (JP 11(1999)-30783 A), a hybrid alignment mode (JP 2005-275342 A), and a homogeneous alignment mode (JP 2005-316407 A).
In particular, the viewing angle control device using homogeneously aligned liquid crystal as disclosed in JP 2005-316407 A has many advantages such as a simple manufacturing process, capability of switching between a wide viewing angle and a narrow viewing angle, and relatively high transmittance in a normal direction. With reference to FIGS. 11A and 11B, a configuration of the viewing angle control device using homogeneously aligned liquid crystal will be described. FIG. 11A is a schematic view showing a state of liquid crystal molecules during a wide viewing angle mode of the viewing angle control device, and FIG. 11B is a schematic view showing a state of the liquid crystal molecules during a narrow viewing angle mode of the viewing angle control device. In FIGS. 11A and 11B, reference numerals 91 and 92 denote a pair of translucent substrates, and reference numeral 93 denotes liquid crystal molecules in a liquid crystal layer sandwiched between the translucent substrates 91 and 92. Arrows denoted by reference numerals 94 and 95 represent polarization-transmission axes of polarizers laminated on the translucent substrates 91 and 92. The polarization-transmission axes 94 and 95 are parallel to each other. Alignment films on the translucent substrates 91 and 92 are subjected to a so-called anti-parallel rubbing treatment in directions substantially parallel to the polarization-transmission axes 94 and 95 and opposite to each other between the translucent substrates 91 and 92.
In this viewing angle control device, in a state where no voltage is applied to the liquid crystal layer, the liquid crystal molecules 93 are arranged such that their long molecular axes are substantially horizontal to the polarization-transmission axes 94 and 95 as shown in FIG. 11A. Thus, even if a view point is inclined horizontally with respect to a panel from a normal direction of the substrates (i.e., in a plane perpendicular to the polarization-transmission axes 94 and 95), no phase difference occurs in the liquid crystal layer, thereby maintaining a wide viewing angle of a display device on which the viewing angle control device is laminated. This state is referred to as a wide viewing angle state.
On the other hand, when a predetermined voltage is applied to the liquid crystal layer; the liquid crystal molecules 93 rise at an inclination angle θ with respect to a substrate surface in accordance with the applied voltage as shown in FIG. 11B. Consequently, if a view point is inclined horizontally with respect to the panel from the normal direction of the substrates, a phase difference occurs in the liquid crystal layer. When the inclination of the view point reaches a certain angle, a black display is caused according to VT (voltage-transmittance) characteristics as shown in FIG. 12. In this manner, a display on the display device is shielded by the black display of the viewing angle control device in a horizontal direction other than the normal direction (front side) of the substrates. This state is referred to as a narrow viewing angle state.